For quite some time now, appreciation of CS Lewis's Narnia books has been bogged down by their Christianity. The biggest gun firing at him lately has been Philip Pullman, whose anti-religiousness at times recalls Kingsley Amis's line about God – "it's more that I hate Him". When I once made a fleeting reference to the debt he owed Lewis in constructing the imaginarium of the Northern Lights trilogy, he wrote me a long, cross and very interesting letter (which I have regrettably mislaid) whose length and crossness suggested that I might have touched a nerve. But as millions of readers can attest, you can still flatly reject Christianity yet enjoy the Narniad (as it's now called) immensely; and new generations continue to do so.

There is something else going on there besides mere Christianity, for all that his friend and colleague Tolkien considered it a hodge-podge. What, for instance, is Father Christmas doing in The Lion, the Witch and the Wardrobe? Why do Silenus, satyrs (aka fauns) and other figures from classical mythology pop up? Surely Lewis was either making it all up as he went along, or he was retreating towards childhood as the result of being humiliatingly trounced by GEM Anscombe in a 1948 debate at the Socratic Club.

Michael Ward thinks he has found an underlying system behind the seven Narnia books, and it can be very easily summarised: each of them represents, or is inspired by, the aspects of each planet in the medieval cosmology (that is, including the sun, the moon, and ignoring everything further away than Saturn, on the reasonable grounds that nothing beyond that planet had been discovered until 1781). So: The Lion, the Witch and the Wardrobe has its Jovian qualities (which helps explain Father Christmas); Prince Caspian its Martian, or rather martial, aspects; The Voyage of the Dawn Treader, perhaps most obviously, is infused with the spirit of the sun, and so on.

You could raise all sorts of objections to this, not least because this "key" has escaped any number of scholars for 50 years, and also not least because there is something not quite respectable about finding secret codes within literary works. (It is often the leper bell of the obsessive or the charlatan.) But Ward, who once attempted to map out the Narniad against Shakespeare's plays, meticulously addresses any objections; he is well aware of the audacity of his claim.

And, as he points out, Lewis was hugely interested in both astronomy and pre-Copernican symbolism; he quotes copiously from his adult science-fiction trilogy, and from his own long, alliterative poem "The Planets". Lewis was also secretive, given to laying false trails, and indeed considered it important to his art to have elements which were deeply buried. Which may strike some as suspiciously convenient for Ward's purpose.

But the whole book is so engagingly written, and so illuminating about medieval symbolism in general, that Planet Narnia is worth reading even if all you are going to do is disagree with it. It also does much to redress the balance of contemporary Lewis criticism, which has, for the most part, concentrated with unremitting hostility on Lewis's reactionary beliefs. (Not just his Christianity, but his perceived racism and sexism, which faults you can, if you're in a condemnatory mood, lay at the door of pretty much any author born before 1940.)

Of course, such a discovery, even if it is true – and, barring the unearthing of new evidence, we cannot know for certain whether Ward is right – wouldn't tell the whole story about the books. Paradoxically, it is this aspect that makes Ward's argument particularly convincing; just as there is more to Ulysses than its Homeric foundations. But that is the wonderful thing about the Narnia books, however flawed or contentious they are to the contemporary Weltanschauung. Even if Ward overstates the importance of the planetary symbolism, it's still a usefully enriching way of looking at them – and gives one an excuse to read them again. Should one be so minded.